Comprehensive Theoretical Investigation of the Mechanism of Methanol to Aromatics Catalyzed by Gallium-Modified FAU Zeolite

The catalytic transformation of methanol to aromatics (MTA) over gallium-modified zeolites has been widely developed as an important technology to produce aromatics such as benzene, toluene, and xylene (BTX) in the petrochemical industry. However, the reaction mechanisms of the MTA process at the atomic level remain unclear but are valuable to experimentally design more efficient catalysts. In this study, the MTA mechanisms on gallium-modified acidic FAU (Ga-FAU) zeolite have been theoretically investigated by a two-layer our Own N-layered Integrated molecular Orbital and molecular Mechanics (ONIOM) method. The MTA process includes methanol to benzene (MTB) and methylation of benzene to hexamethylbenzene (BTH). Their rate-determining steps are the intermolecular proton transfer of cyclohexene for MTB and the methylation step of methylbenzene for BTH. The different elementary steps in the MTB pathway could occur in the following order of reactivity: deprotonation ≥ intramolecular proton transfer > cyclization > intramolecular CH3 shift > methylation > intermolecular proton transfer. In BTH, the deprotonation step is more favorable than the methylation step. In the MTB pathway, the deprotonation, intramolecular proton transfer, and CH3-shift steps are generally entropy-increased, but the cyclization, methylation, and intermolecular proton transfer steps are entropy-decreased. All elementary steps in the BTH pathway are entropy-decreased. Overall, MTB and BTH have almost the same reactivity on the Ga-FAU zeolite. The entire MTA reaction proceeds in the direction of reducing free energies. Differential charge density maps (DCD), local orbital localization maps (LOL), and reduction density gradient maps (RDG) reveal the nature of transition states (TSs) from the viewpoint of electron migration and explain the complicated attractive and repulsive interactions between different molecular fragments in TSs. The wave function analysis suggests that spatial hindrance and van der Waals (VDW) attractive forces generally coexist in the TS fragments. There are clear partial covalent interactions and moderate VDW interactions between the atoms in the forming/breaking chemical bonds.